This invention relates to a method for the recovery of useful products such as oil and gas from hydrocarbon bearing deposits such as oil shale or tar sand by the application of radio frequency energy to heat the deposits. Such techniques are generally classified by the U.S. Patent and Trademark Office in class 166, subclass 248. More specifically, the invention relates to a method and apparatus in which a high power radio frequency transmitter is electrically matched to the varying impedances encountered when conductor arrays, inserted in an earth formation, are employed to efficiently couple radio frequency energy into an earth formation to heat the earth formation.
This country's reserves of oil shale and tar sand contain enough hydrocarbonaceous material to supply this nation's liquid fuel needs for many years. A number of proposals have been made for processing and recovering hydrocarbonaceous deposits, which are broadly classed as "in situ" methods. Such methods may involve underground heating or retorting of material in place, with little or no mining or disposal of solid material in the spent formation. Useful constituents of the formation including liquids of reduced viscosity may be drawn to the surface by a pumping system or forced to the surface by the technique of injecting another substance into the formation.
It has been proposed that relatively large volumes of hydrocarbonaceous formations can be heated in situ using radio frequency energy. These proposals are exemplified by the disclosures of the following patents: U.S. Pat. No. 4,144,935 to Bridges et al., now reissue application Ser. No. 118,957 filed Feb. 2, 1980, which is now U.S. Pat. No. Re. 30,738; U.S. Pat. No. 4,140,180 to Bridges et al., U.S. Pat. No. 4,135,579 to Rowland et al.; U.S. Pat. No. 4,140,179 to Kasevich et al.; and U.S. Pat. No. 4,193,451 to Dauphine.
Embodiments disclosed in these patents call for the heating of oil shale or tar sand with one or a plurality of conductors at least partially embedded in the formation. Embodiments disclosed by Bridges et al. enclose or bound a volume of a formation in an electrical sense with arrays of spaced conductors. One such array consists of three spaced rows of conductors which form the so-called "triplate-type" of transmission line structure similar to that shown in FIG. 1 of this application.
The measurement of electrical and thermal properties of solid hydrocarbonaceous material have been made in the laboratory. See, Joel DuBow, "Electrical and Thermal Properties of Oil Shale of Interest to in-situ Shale Oil Extraction," N.T.I.S. Publication No. PB-267 136 (1977). Variations in impedance of conductor arrays inserted in an earth formation have been predicted. These variations suggest the need for impedance matching techniques to permit maximum power to be transferred to the formation and prevent overloading of the radio frequency transmitter used to provide the power.
Two matching techniques have been proposed by others working in the field. First, the above-cited patent to Dauphine states that the particular impedance of the radiating structure (conductors imbedded in the formation) can be matched by changing taps on a transformer and/or by adding reactive impedances as appropriate to the output of the transformer in accordance with well-known practice. However, continuous matching of a variable load impedance may be unobtainable with the transformer proposed by Dauphine unless a very large number of transformer taps are available. At radio frequencies, typically such transformers have a small number of turns (for example, 10) and the number of taps which can be provided are limited by the number of turns.
A second matching technique was used in a field test in which applicant participated prior to his making of the invention herein described. The field test involved the use of an embodiment of the Bridges et al tri-plate type of transmission line with an "L" matching network such as that shown in FIG. 2a, and described in greater detail below. This network was found to be ineffective to correct for some variations in the load impedance encountered as the formation was heated. An additional correction of impedance mismatch was provided in the field test by changing the effective length of the transmission line to which the network was connected. However, such changes required that the transmitter be shut down and that mechanical changes in the line be made, (e.g., additions or subtractions to the line length), resulting in delays in the application of heat during which the formation could cool. Nevertheless, impedances in certain Smith chart regions could not be matched with the field test apparatus. (This matter is discussed in greater detail in connection with FIG. 2b, below.)
Accordingly, it is a feature of the present invention that impedance changes encountered in radio frequency heating of an earth formation with embedded conductor arrays be compensated without electrical disconnection or shut down of the transmitter coupled to the conductor arrays.
It is another feature of the present invention to provide an impedance matching network which is effective in Smith chart regions corresponding to impedances encountered during the radio frequency heating of an earth formation with a transmission line including conductors at least partially embedded in the formation.
It is another feature of the matching network of the present invention that impedance matching, employing the adjustment of continuously variable electrical elements, be provided in response to variations in the load impedance encountered during the radio-frequency heating of an earth formation with a transmission line including conductors at least partially embedded in the formation.
It is another feature of the matching network of the present invention that impedance matching adjustments be made in an impedance matching network in accordance with simple and unambiguous operating procedures.
These and other features of the invention will become apparent from the claims, and from the following description when read in conjunction with the accompanying drawings.